In recent years, in order to deal with electrification and increased capacity of vehicle on-board equipment such as an electric power steering, an electric air conditioner and the like, a 42V-base vehicle power supply system wherein a 42V-base power supply is added to a conventional 14V-base power supply has been proposed. Since the 42V-base power supply can provide a three-fold voltage relative to the conventional 14V-base power supply, the required current is reduced to one-third, loss is decreased, and harness can be reduced in weight. Additionally, loads of large capacity can easily be dealt with. The capacity of a battery is increased relative to that of a conventional one, and in the future a nickel-hydride battery and/or lithium ion battery may replace the currently used lead acid battery.
However, application of 42V-base power supply to equipment such as a lamp may have an adverse effect of reducing the lifetime, for example, and therefore the conventional 14V-base power supply must also be reserved. Accordingly, recently a power supply system is proposed that includes both a main battery of 42V-base and a sub-battery of 14V-base, as well as a DC/DC converter to boost and step-down the voltage. It is noted that, a battery of 14V-base refers to a battery of which charging voltage is 14V and discharging voltage is 12V. Further, a battery of 42V-base refers to a battery of which charging voltage is 42V and discharging voltage is 36V.
Not being limited to such a power supply system, in a hybrid vehicle wherein the driving force of an engine is assisted by a motor, electric power from a battery of low voltage may be boosted by a DC/DC converter and provided to an inverter of the motor. Often in such a case, in order to reduce the current supplied to the motor for driving the vehicle so that the weight of the harness can be reduced, and also in order to attain high driving force for driving the vehicle, the rated voltage of the vehicle-driving motor is high. On the other hand, in order to increase the voltage of an on-board battery, a multiple of battery cells of about 1.2V must be serially connected. If the multiple of cells being serially connected cannot provide the rated voltage of the motor, the voltage of the battery is boosted by a DC/DC converter to be supplied to the motor via an inverter.
Thus, in an on-board electric circuit, it is necessary to boost and step-down the voltage of a battery, and a DC/DC converter is employed in such a case. Japanese Patent Laying-Open No. 8-214592 (Reference 1) discloses a motor driving apparatus that can realize driving of a motor, regenerative braking and charging of a battery with a simple configuration, and that also can refresh the battery. The motor driving apparatus includes: a driving circuit having at least one arm formed by two serially connected switching elements having flywheel diodes, having its input terminal connected to a battery and having its output terminal connected to a motor for exerting control of conduction over the motor by turning on/off the switching elements; a chopper circuit connected in parallel to the driving circuit and formed by serially connected two switching elements having flywheel diodes; a direct current side reactor connected between a neutral point of the chopper circuit and the battery; and a control circuit provided to exert control so as to turn on/off the switching elements of the driver circuit and chopper circuit, and to cause the chopper circuit to operate as a booster chopper when supplying electric power from the battery to the driving circuit and as a step-down chopper when supplying electric power from the driving circuit to the battery. Specifically, the control circuit exerts control so that the chopper circuit supplies the reference voltage of the battery to the driving circuit when the motor output is low, and that the chopper circuit operates as a booster chopper when the motor output is high.
According to the motor driving apparatus disclosed in Reference 1, when supplying the electric power from the battery via the driving circuit to the motor, the chopper circuit is operable as a booster chopper. Therefore, a voltage higher than the battery voltage can be applied to the motor, and the motor can be driven at higher speed than in the stationary state. Further, when supplying the electric power from the driver circuit to the battery, the chopper circuit is operable as a step-down chopper. Therefore, even when a motor generated voltage or an externally supplied voltage is higher than the battery voltage when the motor functions in regenerative braking mode or the battery is externally charged, the battery can be charged without a damage to circuit elements.
Japanese Patent Laying-Open No. 8-51800 (Reference 2) discloses a control method based on battery voltage estimation wherein control can be continued even when a voltage sensor is involved with abnormality. The control method includes the steps of: determining a motor primary voltage based on a battery voltage and a previous switching pattern; estimating each current value of the motor primary magnetic flux and the motor torque based on the motor primary voltage and a motor primary current; determining each instruct value of the motor primary magnetic flux and the motor torque based on a request output; determining a switching pattern by comparing each estimated current value of motor primary magnetic flux and motor torque with each determined instruct value of motor primary magnetic flux and motor torque; and alternating the motor primary current in accordance with the determined switching pattern to control an induction motor, wherein the battery voltage is detected by a voltage sensor in a normal state, and the battery voltage is estimated based on a load state of the battery when the output of the voltage sensor is abnormal.
According to the control method, when the output value of the voltage sensor is abnormal because of a failure in the voltage sensor detecting the battery voltage or because of a noise superimposed on the output of the sensor, an estimated value of the battery voltage is used. Accordingly, even when the output of the voltage sensor is abnormal, the control based on the fast direct torque control theory. Additionally, even when an error is included in the estimated value of the battery voltage, an unstable control state is not invited so long as the error is not extremely great. Since the battery voltage is estimated based on the load state of the battery, a great error does not occur in the estimated value and a stable control is realized over a wide range.
However, according to the motor driving apparatus disclosed in Reference 1, wherein the chopper circuit is provided in parallel to the inverter circuit so that the chopper circuit operates as a booster chipper when the output of the motor is high and as a step-down chopper when the motor operates in regenerative braking mode, the battery and others may be damaged if abnormality occurs in the voltage sensor during such adjustment of the voltage, since precise voltage control is not exerted. When the voltage is boosted, simple comparison between the voltage value sensed by the voltage sensor on the output side of the DC/DC converter (after boost) and that sensed by the voltage sensor of the battery does not clarify which voltage sensor is abnormal.
Further, according to the control method disclosed in Reference 2, it does not employ a configuration having a DC/DC converter, and instead, the voltage sensor is determined to be abnormal simply based on the relationship between the battery output and the battery voltage, and an induction motor is controlled using an estimated battery voltage in such a case. The abnormality of the battery voltage sensor is sensed only when there is a deviation from a map indicative of the battery output and the battery voltage.
In either case, for example when a voltage sensor on the input side of a DC/DC converter (before boost) is omitted for saving the costs and only a voltage sensor on the output side of the DC/DC converter (after boost) and a voltage sensor of the battery are provided, simple comparison of the voltage values sensed by respective sensors cannot provide precise sensing of abnormality even if either sensor is abnormal.